Abstract: This dissertation focused on identifying the ecological factors controlling tree encroachment into the grasslands on Mount Tamalpais, Marin County, CA. Over the past fifty years, there has been a striking difference in the encroachment of two tree species, Pseudotsuga menziesii and Lithocarpus densiflora. P. menziesii has shown significant encroachment, while L. densiflora has only a few individuals in the grasslands and they are almost exclusively confined to areas underneath the canopies of P. menziesii saplings. To understand how a series of ecological factors were affecting tree encroachment, I conducted experiments examining the effects of seed dispersal, seed predation, seedling facilitation, and mycorrhizal fungi. L. densiflora acorns were abundant in the forest but none were found in the adjacent grasslands, indicating that L. densiflora establishment in the grassland was strongly limited by dispersal. Although seed dispersal of P. menziesii was not measured directly, based on its sapling density in the grassland, it does not appear that its distribution was seed-limited. Seed predation was similar for both species, being much higher in the forest than in the grassland. Field experiments showed that the survival of seedlings of both species was much higher under woody plants already established in the grassland. This strong facilitative effect was mainly due to the amelioration of environmental conditions, particularly water deficit during the summer months. Ectomycorrhizal (ECM) fungal assemblages in the forest were highly diverse and dominated by species that colonized both P. menziesii and L. densiflora. It appeared that in both the forest and under P. menziesii saplings in the grassland, P. menziesii and L. densiflora were likely connected by common mycorrhizal networks. There were also dramatic shifts in ECM composition across the forest-grassland ecotone. The grasslands were dominated by a suite of Pseudotsuga-specific fungi, which may affect the timing of encroachment of P. menziesii and L. densiflora into the grasslands. In conclusion, there is a suite of biotic and abiotic factors that operate in combination to control the patterns and rates of tree encroachment into this northern California grassland. Our ability to predict future changes in forest-grassland boundaries under changing climatic conditions will depend on the basic understanding of the mechanisms that control the distributions of component species.